Because the Moon’s spin axis is nearly perpendicular to the plane of the ecliptic, sunlight is always horizontal at the poles. This results in a thermal environment that is almost constant and is benign relative to any other parts of the lunar surface. Also in the polar regions, topography provides places, such as crater bottoms, where sunlight never strikes the surface. In the perpetual darkness there, equilibrium temperatures are extremely low. At mountain tops and on crater rims sunlight is nearly continuous, though not truly perpetual: The Moon does have seasons because of the 1.5 degree tilt of its polar axis from the ecliptic normal, and also sunlight is greatly reduced (though not totally extinguished, because of refraction through Earth’s atmosphere) for a few hours during a total lunar eclipse. Observations and topographic analysis show that the maximum percentage of surface illumination at any one place over the year is about 70 per cent; if two sites are considered there can be grazing sunlight more than 90 per cent of the time.
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- Perpetual Sunshine, Moderate Temperatures and Perpetual Cold as Lunar Polar Resources
James D. Burke
- Springer Berlin Heidelberg